Grinding and apparatus therefor

ABSTRACT

Apparatus for statistical grinding of material by grinding media maintained in motion by agitation means. The apparatus (1) may be for use on a batch basis or a continuous basis, or (2) may use grinding media of different sizes in a multistep operation, or (3) both. When using grinding media of different sizes, larger grinding media are first used, and then smaller grinding media, in separate vessels. In apparatus for a preferred operation larger grinding media are used in a batch operation, followed by smaller grinding media in a continuous operation.

United States Patent ATTRITOR PREMIXER 1,361,205 12/1920 Vanzandt 241/29 2,168,093 8/1939 Osgood 241/153 X 2,764,359 9/1956 Szegvari 241/15 3,154,255 10/1964 Schulman. 24l/46.17 X 3,275,250 9/1966 Plungvian 241/29 X 3,215,353 11/1965 Goeser 241/15 Primary Examiner-Donald G. Kelly Attorney-Gordon C, Mack INSTALLATION AUTOMATIC CONTROL [72] Inventor Andrew Szegvari 201 Castle B1vd., Akron, Ohio 44313 [21] Appl. No. 869,413 [22] Filed July 14, 1969 7 Division of Ser. No. 591,459, Nov. 2, 1966, Pat, No. 3,493,182, which is a continuationin-part 01 Ser. No. 407,716, Oct. 30, 1964, b vde c e [45] Patented July 13, 1976 [54] GRINDING AND APPARATUS THEREFOR 4 Claims, 9 Drawing Figs.

[52] US. Cl 241/98, 241/137 [51] Int. Cl B02c 17/16 [50] FieldofSeareh 241/153, 15,29,46.17,98,101,137,172,173

[56] References Cited UNITED STATES PATENTS 241,653 5/1881 Harrison 241/153X PREIXIG AMY Tm TWIN ATTRITOR CONTINUOUS MIIITOI FIEGRINNIN uni-mm: no it CONTINUOUI AUTOMATIC PROGZISIIIO MONITOR PATENFED JUL] 3 Ian SHEET 3 OF 6 FIG snmome OF sus susrou 0F souo FEEDS uswe FIRST LARGER THAN SMALLER enmome MEDIA PRO CESS EXPLANATION OF SYMBOLS Any grinder Continuous Grinding Equipment Premixing Device containing activated grinding media continuously charged with components Conventional Premixing Device consisting of an agitator and a vessel Pregrinder (Same as above 6') Converter, changing intermittent into continuous flow Any type of flow with or without a converter Continuous flow Intermittent flow PATENTEBJUUBIQI: 3.592.396

SHEET 5 UF 6 F u 2 u! D 0 Fl 6 6 a 1 ul N G SMALL LARGE C 2 U :a G g Fl G. 6 b v3 F5 SMALL LARGE- C 5 3 E Fl G 6 C u L. W

SMALL LARGE.

an'rcuwlse 5 msnuauflon 3 Fl G 6d. ,32 3% M Burns '1 In SMALL -y- LARGE GRINDING AND APPARATUS THEREFOR This application is a division of my application Ser. No. 591,459 filed Nov. 2, 1966 (now U.S. Pat. No. 3,493,182) which is a continuation-in-part of Ser. No. 407,716 filed Oct. 30, 1964 (now abandoned). The invention as there disclosed is also disclosed in Ser. No. 534,934 filed Jan. 19, 1966 (now U.S. Pat. No. 3,486,705).

The invention relates to apparatus for the grinding of solids suspended in a liquid by contact with grinding media which are activated by agitating means. The grinding media may be balls of different sizes as is known in the art, or pebbles of French flint, as disclosed in Szegvari U.S. Pat. Nos. 2,764,359 and 3,149,789 which describe apparatus known in the industry as attritors; or the grinding media may be smaller particles such as the sand or the like employed in the so-called sand mills," such as disclosed in Hochberg U.S. Pats. Nos.

2,581,414 and 2,855,156, for example.

The advantages of the process of this invention are best understood against a background of the following facts:

1. The operation involving the diminution starts with the combining of solids to be diminuted with liquids. This :initial processing step is usually referred to as premixing and takes place in vessels equipped with a mechanically driven agitator in the absence of grinding media. The acting forces are derived from liquid shear. The nature of liquid shear is such that it is mechanically limited, that is, in a given apparatus its action is limited to a certain amount of dynes per square centimeters of acting area which cannot be exceeded. As a result, the action or liquid shear is limited regardless of duration.

2. The mechanical action of a system of contacting balls is such that the forces available at the contact points could exceed practically all limitations reigning in an apparatus of conventional size. Therefore, the action of such a system on diminution is controlled primarily and only by statistical fact as a function of time.

3. The statistical effect of interaction between a system of activated grinding media and the feed to be diminuted depends first on the size of the interacting grinding media, and second, on the size relationship of these grinding media and the material to be diminuted, and third, on whether the interaction is carried out on a batch or continuous basis.

4. The end result in an industrial diminution process such as described is that the resulting particles have a size range of qualitative characteristics which is usually referred to as a distribution curve. In most instances the industrially required property of this distribution curve is that it should be narrow"that is, the occurring particle size range should be within the narrowest possible limits.

The control of the distribution curve is described in the following:

The width" or range of the distribution curve is narrower in batch-type operations (FIG. 4) and much wider, or flatter, in continuous operations because the probability of something happening, which is different from the average, is much smaller in the closed-circuit batch operation, where there is chance processwise of each cubic inch being like every other cubic inch, which is nothing else but the condition to have a narrow" or sharp distribution curve. The processing drawback of this statistical procedure is that it takes time; much longer than necessary to obtain the flatterdistribution curves of a continuous statistical process.

Producing the largest amount of fine particles, or in statistical terms, the maximum area under the fine end of the distribution curve (FIG. 3): This is obtained with small or smallest grinding media, high agitator shaft speeds and continuous grinding equipment. The cause is the statistical probability of contacts between small grinding media being much larger than between large grinding media, and obviously in order to produce a large number of fine particles we have to have the largest number of contacts.

Elimination of the larger! or coarsest particles: That is reduction of the area of the distribution curve under the large end A. The action of large grinding medium and coarse feedwould eliminate the large particles without producing fines.

B. A large grinding medium on a fine feed would have no required effect on the absent large particles; on the other hand, it does not have any statistical chance to produce fineness effectively.

C. A small grinding medium on a coarse feed has very little statistical chance to eliminate the large particles but it will effectively made small particles from the small fraction, thus resulting in a very wide distribution curve.

D. A small grinding medium on a fine feed .will effectively produce the largest amount of small particles.

5. [t is impractical to combine large and small grinding media in the same grinding tank. There are two causes for this: One cause is that the overriding momentum of the larger grinding media prevents the activity of the smaller grinding media; and the second cause is that larger grinding media segregate from the smaller grinding media in amechanically undesired way.

6. It is a significant'fact that because of-statistical reasons the rate of diminution obtained as a result of the interaction between a system of activated grinding media'and the feedis such that it proceeds first much faster-in fact, at a surprisingly fast rate-and this rate slows down more andmore during the subsequent processing.

7. There is a significant difference in the statistical procedure of the diminution between (1) the case where diminution is effected in a closed circuitthat is, in a batch operation and (2) a continuously progressing through flow or block flow" type of continuous operationxln the first case the condition obtained, i.e. the diminution of the particles,.i s continuously equalized so that the size of the particlesis the same throughout the vessel at any given time, while in the second case the diminution progresses from one end of-the vessel or operation to the other, and the size of the particlesis continuously being diminuted without any diminuted condition being equalizedwith a previous state.

8. The consequence of the facts set forth in No. 3 above, and part of No. 7 above, is that the best statistical progressiin diminution by a system of activated grinding media is obtained by using small grinding media interacting with a-feed whichis ground to a size as small or fine as possible. Particularly, this is feed. The above results from the use of apparatus which ac,

complishes two things: (a) a very good premix" is provided, and (b) the process is carried out in series fashion-that is, the diminution result initiated in a first unit is continued in a second unit containing smaller grinding media, resulting in .-a series" type of processing. By premix" is meant the mixing of the solids and liquid previous to feeding. them. into the continuous diminuting apparatus. Some diminution may occur during the premixing. While premixing is essential for continuous diminution units, this is not the case with batch operations where the solids and liquid can be fed without "premixing.

9. The mechanical or, more precisely, the kinetic behavior of agitating grinding media and their interaction with the feed to be diminuted is different with large media from what it is with small media. Because of this difference inbehavior; the large media perform best in batch-type operations and cannot be used satisfactorily in continuous apparatus, while small media perform bestin continuous apparatus,.which applies particularly to operations in which the suspension .of the feed rises vertically.

The invention in its various adaptations utilizesone or more of the followingnovel principles:

A. Advantages are gained by the superimposing of consecutively applied diminution process based on activated grinding media yielding different size distribution statistics, such as the following:

I The statistics ofa batch-type diminution operation;

2. The statistics of a continuous diminution operation;

3. The statistics prevailing when using large grinding media in relationship to the feed.

4. The statistics prevailing when using small grinding media in relationship to the feed.

5. The interrelated statistics when using consecutive units in case of continuous operations.

B. Advantages of extending the use of activated grinding media to the premixing operations and thereby superimposing the distribution obtained on the distribution of subsequent grinding operations.

C. Advantages of combining the operation involving batchwise processing with subsequent continuous operation by means of a converter; by converter is means apparatus to convert pulsating flow into continuous flow.

The accompanying drawings explain and illustrate the invention. In these drawings:

FIG. 1 is a distribution curve in which fineness measured in Hegman gauge is plotted on the vertical axis and time is plotted on the horizontal axis. (The Hegman gauge is used commercially in the paint and other industries, and ranges from Hegman to Hegman 8. The larger numbers indicate greater fineness.)

FIG. 2 is a family of graphs, wit the vertical axis measuring premixing time in minutes required to arrive at a certain Hegman fineness (using activated grinding media), and the horizontal axis indicating the grindability" of the particular slurry in some practical units thus indicating the grindability of-the particular dispersion in a practical yardstick. For such a yardstick we use the ordinarily well-known grinding time in conventional ball mills, expressed in hours.

FIG. 3 is a flow sheet of various processing operations which are within the scope of the invention;

FIG. 4 is a representation, largely schematic, of apparatus for carrying out Process D of FIG. 3;

FIG. 5 is a representation, largely schematic, of apparatus for carrying out Process E of FIG. 3; and

FIGS. 6a to 6d are distribution curves. FIG. 6a is a representative distribution curve of the particle sizes ofa solid powder. FIG. 6b shows in dotted lines the effect of diminution using relatively large grinding media (the solid line showing is the original FIG. 6a). FIG. 60 shows in solid lines the effect of diminution with relatively small grinding media, the solid line showing is the original curve of FIG. 6a. FIG. 6a' shows the curve of FIG. 6a to the right, and to the left shows the effect of a batch and a continuous operation carried out to the same particle size.

FIG. 1 shows the fineness expressed in Hegman gauge grades, as a grinding progresses, which applies generally to the effect of activated grinding media on solids in liquids. The horizontal axis is scaled so as to indicate full grinding time of l O0,"regardless of how long the grinding continues. This applies to all materials and equipment. The percentage means simply the percentage of the full grinding time expressed in the same time scale. The graph shows the extremely rapid rise in the amount of fine particles obtained during the first few percent ofthe elapsed total grinding time.

FIG. 2 indicates that when premixing in a batch-type premixer, containing activated grinding media such as described for use in Process D below, one can obtain 3 to 5 Hegman grade fineness in a relatively few minutes, mostly less than 10 minutes, with any slurry with usual average grindability, such as finished in a conventional ball mill within two days.

FIG. 3 shows a series of operations schematically in each of which a slurry of solid material in a liquid is subjected to two or more grinding steps by activated grinding media, in the first of which larger grinding media are employed and in the second of which smaller grinding media are employed.

PROCESS A: This is a general process illustrated schematically, in which any two types of grinding equipment containing activated grinding media are used with the grinding media in the first vessel of larger size than those in the second vessel. The suspension produced in the first vessel is conveyed continuously or intermittently to the second vessel. There may or may not be some type of converter to which the slurry is subjected between the operations carried out in the two grinding vessels.

The processes following Process A are selected as presently appearing to have the greatest commercial possibilities Processes C and D are particularly adapted for use in the fine grinding of pigments for paints, inks, etc., and other processes may be particularly adapted for the grinding of other solids including drugs and inorganic materials.

PROCESS B: Process using a premixing device containing activated grinding media which discharges into continuous grinding equipment, with or without an intermediate converter.

PROCESS C: Process using a premixing device containing activated grinding media which is continuously charged with metered amounts of solid and liquid. The premixer is discharged continuously into continuous grinding equipment.

PROCESS D: Solid and liquid components of a slurry are combined conventionally and fed into a batch pregrinding device containing activated grinding media and by way of the instrumentality ofa converting mechanism feeding into a continuously grinding unit. An example for this process is the large-scale production of the so-called white base of the paint industry consisting essentially of a titanium dioxide pigment suspended in the properly formulated slurry. In this operation the pigment, as illustrated in FIG. 4, which may be of a size of about 325 mesh, will, for example, be fed once a day with the suspending medium to a premixer l which'may be a 600-gallon tank. This will be circulated by the pump 2 through the tank I and part of this will be directed through the action of an automatic monitor 3, intermittently, to the first or the second of the twin batch pregrinders 4 and 5 containing activated grinding media of a size between 3/16 to 3/8-inch. These pregrinders may each have a capacity of about 40 gallons and will grind the pigment to an average Hegman gauge of 4 to 5. The finished premix is metered automatically and continuously by way of the controlling monitor 3 and pump 6 into the continuous grinding unit 7 which contains grinding media in the size range between [/64 and l/8 inch. This unit 7 may be of the type illustrated and described in Szegvari US. Pat. No. 3,149,789. The solids are ground to an average Hegman fineness greater than 7.

PROCESS E: As illustrated in FIG. 5, the solid and liquid components of the slurry are charged into the batch premixer 10 containing activated grinding media, and pumped intermittently by pump 11 into an intermediate premix accumulator 12. The size of this accumulator is such that it can provide continuous feeding through a metering pump 13 to the continuous grinding device 14, requiring at least two overlapping periods of the premixer 10. The device 14 may be of the type described in Szegvari US. Pat. No. 3,149,789. The premixer 10 may be, for example, of 40-gallon capacity, and using balls 3/8 inch in diameter will produce a premix of a size of about Hegman gauge 2%1/2 to 4%1/2 every l0 minutes. In the continuous grinding device 14, smaller grinding media will be used in the range of H64 inch to H8 inch and the processed slurry will be ground to a Hegman gauge of 7%1/2 to 8.

PROCESS F: Slurry premixed in any type of premixing device is metered into a continuous grinding device from which a product issues into a second continuous grinding device. These continuous grinding devices contain grinding media in the range between US inch and H64 inch; the first grinding device having grinding media in the range between l/8 inch and 3/32 inch and the second continuous grinding device having grinding media in the range between 3/32 inch and H16 inch. The first and second grinding devices may have additional differences, namely, the rpm. of the first grinding device may be between the range of I and 600 r.p.m., while in the second grinding device, the r.p.m. may be in the range between 150 and 1,000 r.p.m. Apparatus such as described in Szegvari US. Pat. No. 3,149,789 may be used for each of these continuous grinding units. This process is adapted for the fine grinding of powders, drugs, etc. to an average particle size in the range of l to 2 microns.

PROCESS G: This process is similar to Process F, with the exception that the components of the liquid slurry are charged into a premixing device containing activated grinding media, which device completes the premix in requested time intervals; and the slurry is passed through a converter C by continuous flow into the continuous grinding device. Such apparatus is used in the processing of ferrites, starting with a material between 200 and 325 mesh and finishing at the average particle size of 1 micron. Although various types of premixers may be used in Process B, C, E and G, a preferred type of premixer contains activated grinding media. It may be such a premixer as is described in the following and known as an attritor premixer. In many respects this premixer resembles the grinding device disclosed in Szegvari US. Pat. No. 2,764,359 but generally it will not contain as many grinding media as used in a commercial grinding operation. The grinding media will usually be of a size of at least between A inch and inch. The top of the tank will conveniently be equipped with an apron to facilitate manual loading with solid and liquid feed material and it will be equipped with a discharge conduit suitable for continuous or intermittent discharge of the slurry to a flow converter or a continuous grinder or other equipment as required. Discharge opening is provided for the discharge of the premixed slurry and the grinding media. Exposed parts of the drive are protected from contamination by ingredients floating in the air as a result of the loading process. All apparatus details which serve the purpose of finishing the grinding to ultimate fineness, such as cooling means and specific tank-handling equipment are omitted.

In the processes described, a converter" is used for the purpose of changing intermittent discharge from the premixing devices containing activated grinding media into a continuous flow to feed continuous grinding apparatus.

FIGS. 5 indicates the use of an intermediate premix flow converter, the regulation of which is manually controlled, following the preferred type of premixer, an attritor premixer, just described.

FIG. 4 shows an apparatus where a large quantity of solid and liquid, for instance, the entire supply used in one shift, is combined in a container of sufficient size; the size of the container might be between 200 and 800 gallons. These containers are equipped with a conventional agitator to assist in the distribution of solids in the liquids and to keep the distributed solids from excessive sedimentation. No grinding media are used. The control of the flow which involves charging the twin premixers containing activated grinding media, and the subsequent intermittent discharging of each of these premixers into the continuous grinding equipment, is controlled by an electric monitoring device which acts on the four valves to control the intermittent charging and the discharging of the' twin system. The controls of the monitor are effected by conventional timing and level control apparatus.

The invention is disclosed in the claims which follow.

Iclaim:

1. Apparatus for grinding a solid to a size of a few microns in a liquid which contains substantially spherical grinding media and means for activating the same, which apparatus comprises two units each equipped with grinding media and means for activating the same, the equipment of the first unit being adapted for batch operation and equipped with substantially larger grinding media than those with which the second unit is equipped and the equipment of the second unit being adapted for continuous grinding and equipped with substantially smaller grinding media than those with which the first unit is equipped, and means for conveying the solid and liquid resulting from grinding in the first unit into the bottom of the second 'unit.

2. The apparatus of claim 1 in which a converter is interposed between the two units which converts intermittent flow from the first unit to continuous flow to the second unit.

3. The apparatus of claim 1 in which the first unit comprises two pieces of apparatus each operated on a batch basis and the second unit is connected to be fed alternately from said two units, and there is a premixer in which the solid is suspended in the liquid, and means is provided to discharge from the premixer alternately to the two pieces of the first unit.

4. The apparatus of claim 1 in which the first unit comprises two pieces of apparatus each operated on a batch basis and the second unit is connected to be fed alternately from said two pieces.

Patent No.

Inventor(s) PO-105O Dated July 13 1971 Andrew Szegvari It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line Column 3, line Line Column 4, line Line Signed and.

(SEAL) Attest:

EDWARD M.FLETGHEH,JR.

sealed this 18th day of January 1 972.

Attesting Officer- ROBERT GOTTSCHALK Acting Commissioner of Patents 

1. Apparatus for grinding a solid to a size of a few microns in a liquid which contains substantially spherical grinding media and means for activating the same, which apparatus comprises two units each equipped with grinding media and means for activating the same, the equipment of the first unit being adapted for batch operation and equipped with substantially larger grinding media than those with which the second unit is equipped and the equipment of the second unit being adapted for continuous grinding and equipped with substantially smaller grinding media than those with which the first unit is equipped, and means for conveying the solid and liquid resulting from grinding in the first unit into the bottom of the second unit.
 2. The apparatus of claim 1 in which a converter is interposed between the two units which converts intermittent flow from the first unit to continuous flow to the second unit.
 3. The apparatus of claim 1 in which the first unit comprises two pieces of apparatus each operated on a batch basis and the second unit is connected to be fed alternately from said two units, and there is a premixer in which the solid is suspended in the liquid, and means is provided to discharge from the premixer alternately to the two pieces of the first unit.
 4. The apparatus of claim 1 in which the first unit comprises two pieces of apparatus each operated on a batch basis and the second unit is connected to be fed alternately from said two pieces. 